Conventionally, a synchronous machine, such as a permanent magnet motor or a reluctance motor, is controlled such that an armature current vector is oriented in a fixed phase direction with respect to a rotor. For example, a conventional permanent magnet motor is controlled such that an armature current vector is oriented in a direction perpendicular to an axis of magnetic flux produced by each permanent magnet of a rotor and the magnitude of the armature current vector becomes proportional to a desired torque.
On the other hand, while synchronous machines which utilize reluctance torque produced by saliency of the rotor are increasingly used in recent years, it is known that the magnitude of the armature current vector is not proportional to a torque produced by these motors and, thus, it is difficult to control the motor torque with high precision by conventional control methods. Also, when the turning speed of a permanent magnet motor increases, an armature voltage increases due to a voltage induced by magnetic flux produced by permanent magnets and eventually exceeds a voltage that a control apparatus can output. To prevent this inconvenience, the permanent magnet motor is operated by field-weakening control in which an armature current vector having a negative current value (or field-weakening current) is produced in a direction of an axis of the magnetic flux produced by each permanent magnet for reducing interlinked armature flux. Even when the field-weakening current is kept unchanged, however, the armature voltage varies with changes in output torque. It has therefore been difficult to control the armature voltage to match a desired value under conditions of varying output torque by the conventional control methods.
A technique described in patent document 1 is a known example of means for solving the aforementioned problem. The technique disclosed in patent document 1 is such that an interlinked armature flux command and an armature current (torque current) command perpendicular thereto are generated so that a synchronous machine produces a desired torque and armature voltage, a current (magnetizing current) command oriented in a magnetic flux axis direction is determined so that the interlinked armature flux obtained by calculating magnetic flux from the armature current coincides with the aforementioned flux command and, then, current commands oriented in d- and q-axis directions of a rotor are generated from the torque current command and the magnetizing current command.
Patent document 1: Japanese Patent Application Publication No. 10-243699 (paragraphs 0038-0040, FIG. 1, FIG. 3)